Conventionally, in this type of ultrasonic flow meter, a pair of ultrasonic transducers are provided on a measurement fluid passage to face each other, and a fluid flow is measured based on a propagation time between the ultrasonic transducers. A measurement fluid passage section attached with the ultrasonic transducers is connected to an ultrasonic measuring circuit section which measures a propagation time and calculates a fluid flow via lead lines (see e.g., Patent Literature 1).
In another example, a measurement fluid passage is constructed as a unit in such a manner that ultrasonic transducers mounted to a measurement fluid passage section is tightly sealed (see e.g., Patent Literature 2).
FIG. 7 shows a conventional flow meter device disclosed in Patent Literature 1. As shown in FIG. 7, this flow meter device includes a measurement fluid passage 3 provided inside a gas chamber 1 via a separating wall 2 separating an inlet 1a and an outlet 1b from each other, ultrasonic transducers 4 and 5 provided to face other to sandwich the measurement fluid passage 3, a controller 6 provided outside the gas chamber 1, and lead lines 7 coupling the ultrasonic transducers 4 and 5 to the controller 6.    Patent Literature 1: Japanese Laid-Open Patent Application Publication No. 2003-97993    Patent Literature 2: Japanese Laid-Open Patent Application Publication No. 2008-227658